Hughes, R.M., S. Howlin, and P.R. Kaufmann. 2004. A biointegrity
index for coldwater streams of western Oregon and Washington.
Transactions of the American Fisheries Society 133:1497-1515.
WED-00-036

We developed, tested, and applied an index of
biological integrity (IBI) for fish and amphibian assemblages in
coldwater streams of the Oregon and Washington Coast Range. A
probability sample of 104 wadeable sites was quantitatively sampled
for fish and amphibian assemblages as well as physical and chemical
habitat from 1994 to 1996. Natural gradients and anthropogenic
disturbances were assessed by examining digital data for
catchment-scale road density and vegetation cover, along with
site-scale physical and chemical habitat data. A set of 109
candidate metrics was evaluated for variance properties, redundancy,
and responsiveness to multiple measures of disturbance, resulting in
the selection of eight metrics for the index. The IBI itself was
subsequently evaluated for variance and responsiveness to
disturbance, then compared against an independently selected set of
101 reference sites that had minimal anthropogenic disturbance. Our
IBI was fairly precise, with an among-stream variance-residual
(sampling time, measurement error, and crew error) ratio of 4.0
(indicating a theoretical maximum correlation of 0.80 between the
IBI and a predictor variable with a similar ratio). The IBI was
significantly correlated with multiple estimates of anthropogenic
disturbance, and reference sites had significantly higher IBI scores
than the nonreference sites. Low IBI scores were associated with low
bed stability, instream cover, and riparian cover and structural
complexity but high percent fine substrate, road density, and human
disturbances of riparian areas. Applying this IBI, we assessed fish
assemblage condition in the Coast Range, inferring our results to
all mapped (1:100,000-scale) wadeable streams in this region. Using
2 SDs from the reference sites mean as a biological criterion, 45%
of stream kilometers (10,646 km) were classified as impaired. High
IBI scores clustered near national parks and wilderness areas. Our
results indicate that minimally disturbed regional reference sites
and probability survey designs, which produce statistically
representative sites and inferences with known confidence bounds,
offer considerable advantages over least disturbed reference sites
and hand-picked survey sites for making regional ecological
assessments.